In the development or production of gas or oil wells it frequently becomes necessary to seal or plug the well casing at a desired depth. This sealing or plugging forms a barrier in the well bore to isolate upper and lower sections of the bore from each other. After the plugging has taken place, a pressure differential across the plug can exist and can vary from a few pounds per square inch to several thousand pounds per square inch. It is common for such plugs or sealing devices to be run into the well bore or casing on a wireline and then expanded or locked into place by means of generating a high pressure gas within the plug, or by generating a high pressure gas in a tool used to stroke the plug. Generation of the high pressure gas can be typically by the burning of a propellant, either in the plug itself or in a separate, but attached, firing chamber.
Currently known sealing devices typically use a series of concentric cylinders or chambers which move axially relative to one another to drive slip segments outwardly against the well casing. A feature commonly found in such devices is an element such as a shear pin which can shear to release the sealing device from the wireline after sealing has taken place. It is typical among such known devices to either shear the shear pin by jerking upwardly on the wireline after setting the sealing device or to shear the shear pin after setting of the sealing device by means of upsetting the shear pin structure through continued expansion of the propellant gas after setting of the sealing device is accomplished. Regardless of which of these two methods is used, it is a common occurrence to have a bridging plug or sealing device which does not set and seal tightly against the well casing and in which the shear pin has failed to shear, leaving the partially set sealing device attached to the wireline. It can then be very difficult to retrieve the wireline or the sealing device from the casing, and such removal can involve considerable expense to the operator.
It would be desirable to design a well sealing device which insures positive release from the wireline and which at the same time insures positive sealing against the well casing.